The present invention relates to a method and an apparatus for controlling a motor, by which, for example, a synchronous motor that serves as a power source of an electromobile is controlled.
Based on the establishment of the vector control theory, it has been common practice that d-axis current (hereinafter, referred to as I.sub.d current) is controlled generally for more efficient control (for example, see "Flux-Weakening Regime Operation of an Interior Permanent-Magnet Synchronous Motor Drive" by Thomas M. Jahns of IEEE Transactions on Industry Applications, Vol. IA-23, No. 4, July/August 1987, and European Patent Publication No. 0 503 879 A2).
Recently, by making aggressive use of the I.sub.d current for the purpose of the control of high-speed range rotation of a motor, the field-weakening control is beginning to be introduced, in which the motor is weakened in effective magnetic flux by the flow of the I.sub.d current so as to be enabled to perform high-speed rotation.
FIG. 21 is a schematic arrangement view of an electromobile. In FIG. 21, there are shown a car body 51, front wheels 52, rear wheels 53, a motor 54, a transmission 55, and a battery 56. A controller 57 receives inputs of an acceleration signal and a brake signal. In this electromobile, the motor 54 is operated with the battery 56 used as an energy source, the drive force of the motor 54 being transferred to the rear wheels 53 via the transmission 55. The motor 54 is controlled by the controller 57.
For electromobiles, the following advantages are obtained by controlling the d-axis current.
Assume that the motor cannot increase its rotational speed beyond a certain value (for example, 5000 rpm). As the electromobile speed becomes higher, the rotational speed of the motor must be made higher in linkage with the wheels. However, since the motor, having reached 5000 rpm, could not increase its rotational speed any more, its transmission is exploited to increase the speed.
This being the case, the d-axis current control (field-weakening control) herein proposed, if effected, allows the motor, which only could rotate up to 5000 rpm without field-weakening control (solid line A.sub.1) to be rotatable up to, for example, 10,000 rpm by virtue of effecting the field-weakening control (solid line A.sub.2), as shown in FIG. 22. As a result, it can be expected to provide an electromobile without any transmission. In FIG. 22, reference characters TT.sub.1 and TT.sub.1 /2 denote torques while the motor is at rest.
Also, further advantages can be expected, such as reduction in the cost due to the omission of the transmission, and improvement in efficiency due to the effect of the reduction in weight.
Moreover, if proper control is performed, more efficient control is effected, which means that limited energy (battery) serves for more efficiency, such advantages as extended running distances can be expected.
It is noted that if a motor that is rotatable up to 10000 rpm as the motor characteristic was created, the motor would be too large in size, with a greatly increased weight, to be adopted for the electromobile.
However, almost no means has been published for actually realizing the field-weakening control.